posted on Oct, 23 2019 @ 07:27 PM
originally posted by: Phage
a reply to: galadofwarthethird
It's all about the Reynolds number.
A man sized airplane or rotorcraft on Mars will be a problem.
Yes and no.
Yes, the Reynolds number is a very important factor determining the performance of airfoil sections on very small Mars airplanes and helicopters.
Airfoil sections with low Reynolds numbers on wings, tails, propellers, and rotors basically can't develop lift coefficients as high as airfoil
sections on aircraft with larger Reynolds numbers. That usually means that small Mars airplanes and helicopters have to have proportionately larger
wing/rotor blade areas compared to their larger counterparts and/or maintain higher flight speed and tip speed. But there are a couple of finer
points that need to be considered, also. At normal Reynolds numbers that you would associate with a Cessna or anything larger, when you want to
increase the wing area, you would normally increase the span because that results in a higher L/D and more efficiency. With low Reynolds number
designs, if you maintain the chord length the same and increase the span, you still can't pull high lift coefficients because the section Reynolds
number is based on the chord length. On the other hand, if you keep the wing span constant and increase the chord length you can pull higher lift
coefficients and gain performance that way. This is why when you run optimizations codes on very small aircraft (i.e., UAVs), you often end up with
short, wide wings. That's why small insects like bees and flies end up that way also.
Another consideration that usually only applies to Mars airplanes that are deployed mid-air from parachutes is the fact that even normal sized wings
and tails that would have relatively large Reynolds numbers at their design flight speed may be required to operate at extremely low Reynolds numbers
before they pick up speed. One test that I worked on dropped a Mars airplane prototype, nose down, from a balloon at 100,000 feet and had to
immediately begin a pull up maneuver to avoid overspeeding. The first 10 seconds of flight began right on the hairy edge of the wings and elevator
surfaces stalling, before the Reynolds number increased to the point where we had a normal amount of performance margin. The curve of maximum lift
coefficient attainable vs Reynolds number is very nonlinear when you are near the zero point.
No, there's no particular reason a human-sized (or larger) Mars airplane or rotorcraft can't be designed, as long as there's some way to deliver it to
Mars. As the aircraft gets larger, the Reynolds number sensitivity goes away and the vehicle becomes a pretty normal design problem. Many of us in
the Mars airplane community think that once human exploration of Mars begins, heavier-than-air craft will be used to help exploration pretty much the
same way it is used on Earth. That could include robotic drones for collecting the equivalent of ISR and VTOL air taxis and bush planes. Once upon a
time I briefly worked with the guy who did the aerodynamic design of the 2020 microcomputer on a concept for a stowable rotor system to land very
heavy (> 10 ton) payloads on the Martian surface. There didn't seem to be any reason it wouldn't work.
Aerodynamics is the same everywhere in the Solar System.